If you just want the audio of the youtube video, you can issue the following command to get it. The default is 128kbps. If you want to keep the video file too after the audio has been extracted, you can use the -k option (keep).
Hence all together to download the video http://www.youtube.com/watch?v=22UiGcxrbu8 and extract the audio in mp3 format with a bitrate of 176kbps and keep the video, you can use the command: